The invention relates to an electrically controlled fuel injection system for internal combustion engines. It relates particularly to engines which employ external ignition and which include at least one, preferably several, electromagnetically actuatable fuel injection valves, at least one of these valves associated with each of the engine cylinders. The fuel injection system for these engines would normally be connected with a control multivibrator circuit connected to a final control element. The control multivibrator includes a capacitor whose discharge time determines the switching state of the circuit and hence the duration of fuel injection. Constant current sources serve to supply current for charging and discharging the capacitor in the multivibrator and the behavior of these constant current sources is dictated on the basis of the air flow rate and the rpm of the engine.
A known fuel injection system of this general type includes a power output stage in series with the magnetic windings of the injection valves and includes at least one semiconductor switching element. Connected ahead of this power output stage is a so-called dividing control multivibrator which switches in synchronism with the crankshaft rotations of the engine while the injection valves are opened simultaneously. This multivibrator is held in its unstable state during the discharge time of a capacitor for defining the fuel injection duration. This capacitor is charged prior to discharge during a predetermined angular path of the crankshaft while the discharging process is defined by the air quantity supplied to the engine. For this purpose, the induction tube of the internal combustion engine preferably includes an air flow rate meter which generates an electrical variable associated with a time average of the air flow rate for steering the charging and discharging process of the capacitor.
This known circuit is designed to be used with injection valves which receive fuel under constant pressure so that the fuel injection system merely defines the opening time of the injection valves and thereby defines the quantity of fuel fed to the cylinders. This fuel injection system receives a trigger pulse during each crankshaft rotation, preferably from the ignition system of the engine, and this pulse is fed to a pulse shaping circuit and, if necessary, a frequency divider circuit and is supplied to the above-mentioned so-called dividing control multivibrator circuit whose output pulse substantially defines the injection duration for the fuel injection valves. This circuit, which will be designated as a control multivibrator circuit in the following text, may be further associated with a pulse extension circuit as well as a voltage correction circuit so that additional conditions may be considered, for example a dependence on the throttle valve position, a fuel enrichment during starting or post-starting as well as a warm-up enrichment.
In principle, the control multivibrator circuit is so constructed that the duration of the pulses which are generated and thus the fuel injection duration depend, preferably, on a control voltage which depends on the air flow rate and which is preferably adjusted by means of a potentiometer, as well as on the rpm. The output pulse t.sub.p must be a signal proportional to the air flow rate Q which is then divided by the number of suction strokes in the time interval, i.e., by the rpm of the crankshaft, so as to obtain an injection signal which corresponds to the air quantity for each suction stroke. In this manner, an approximately correct stoichiometric mixture is obtained in the entire air flow rate and rpm domain. The generation of the injection pulses t.sub.p and the appropriate division by the rpm is performed in the control multivibrator circuit.
The above described circuit may, however, introduce certain difficulties due to the effects of extraneous and disturbing voltages that may occur in the vehicle which carries the internal combustion engine and these voltages may have deleterious effects on the operation of the control multivibrator circuit. For example, the potential at the base of the trigger transistor is defined by the charge of the capacitor connected to it whereas the emitter of this transistor is exposed to extraneous induced voltages, due, for example, to the ignition system, the alternator and several other switches. Under certain unfavorable circumstances, extraneous simulated trigger pulses may occur because the charge on the capacitor is unable to adapt rapidly enough. Another possible advantage is due to the accumulation of individually very small but finite propagation times of the signals in the feedback networks of the control multivibrator. These delays may lead to an error in division because the pulse time t.sub.p appears to be shortened by these rpm independent times.